Layout of xbars with multiple sub-resonators in series

ABSTRACT

An acoustic filter device includes a transversely-excited film bulk acoustic resonator (XBAR) including a plurality of sub-resonators, and conductors connecting the plurality of sub-resonators in series between a first node and a second node. At least one of the conductors connects two of the plurality of sub-resonators and has a shape that minimizes an area of the at least one conductor.

RELATED APPLICATION INFORMATION

This patent claims priority from Provisional patent application 63/167,506, filed Mar. 29, 2021, entitled IMPROVED LAYOUT OF XBARS WITH MULTIPLE SUB- RESONATORS IN SERIES.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.

BACKGROUND

Field

This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to filters for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a pass-band or stop-band depend on the specific application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application.

RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements.

Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonly incorporate acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies and bandwidths proposed for future communications networks.

The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. Radio access technology for mobile telephone networks has been standardized by the 3GPP (3^(rd) Generation Partnership Project). Radio access technology for 5^(th) generation mobile networks is defined in the 5G NR (new radio) standard. The 5G NR standard defines several new communications bands. Two of these new communications bands are n77, which uses the frequency range from 3300 MHz to 4200 MHz, and n79, which uses the frequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79 use time-division duplexing (TDD), such that a communications device operating in band n77 and/or band n79 use the same frequencies for both uplink and downlink transmissions. Bandpass filters for bands n77and n79 must be capable of handling the transmit power of the communications device. WiFi bands at 5GHz and 6GHz also require high frequency and wide bandwidth. The 5G NR standard also defines millimeter wave communication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is an acoustic resonator structure for use in microwave filters. The XBAR is described in patent U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer, or diaphragm, of a single-crystal piezoelectric material. The IDT includes a first set of parallel fingers, extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm. XBAR resonators provide very high electromechanical coupling and high frequency capability. XBAR resonators may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are well suited for use in filters for communications bands with frequencies above 3 GHz. Matrix XBAR filters are also suited for frequencies between 1GHz and 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view, two schematic cross-sectional views, and a detailed cross-sectional view of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an alternative schematic cross-sectional view of an XBAR.

FIG. 3 is a graphic illustrating a primary acoustic mode in an XBAR.

FIG. 4A is an equivalent circuit model of an acoustic resonator.

FIG. 4B is a graph of the admittance of an ideal acoustic resonator.

FIG. 5 is a schematic circuit diagram of a band-pass filter using acoustic resonators in a ladder circuit.

FIG. 6A is a schematic diagram of a layout of an XBAR with two sub-resonators in series.

FIG. 6B is a schematic circuit diagram of the two sub-resonators in series of FIG. 6A.

FIG. 7 is a graph of the simulated performance of the circuit with the two sub- resonators of FIG. 6.

FIG. 8 is a schematic circuit diagram of a cascaded shunt resonator showing a layout of sub-resonators and conductors connecting the sub-resonators in series.

FIG. 9 is a graph of the simulated performance of the cascaded shunt resonator of FIG. 8.

FIG. 10 is a schematic diagram of an improved layout of an XBAR with sub- resonators in series.

FIG. 11 is a flow chart of a process for making an XBAR filter device.

FIG. 12 is a flow chart of another process for making an XBAR filter device.

Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view, orthogonal cross-sectional views, and a detailed cross-sectional view of a transversely-excited film bulk acoustic resonator (XBAR) 100. XBAR resonators such as the resonator 100 may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suited for use in filters for communications bands with frequencies above 3 GHz. The matrix XBAR filters described in this patent are also suited for frequencies above 1GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on a surface of a piezoelectric plate 110 having substantially parallel front and back surfaces 112, 114, respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. The piezoelectric plate may be Z-cut (which is to say the Z axis is normal to the front and back surfaces 112, 114), rotated Z-cut, or rotated YX cut. XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to a surface of the substrate 120 except for a portion of the piezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140 formed in the substrate. The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm” 115 due to its physical resemblance to the diaphragm of a microphone. As shown in FIG. 1, the diaphragm 115 is contiguous with the rest of the piezoelectric plate 110 around all of a perimeter 145 of the cavity 140. In this context, “contiguous” means “continuously connected without any intervening item”. In other configurations, the diaphragm 115 may be contiguous with the piezoelectric plate around at least 50% of the perimeter 145 of the cavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back surface 114 of the piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120 or attached to the substrate in some other manner. The piezoelectric plate 110 may be attached directly to the substrate or may be attached to the substrate 120 via one or more intermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity 140 may be a hole completely through the substrate 120 (as shown in Section A-A and Section B-B) or a recess in the substrate 120 under the diaphragm 115. The cavity 140 may be formed, for example, by selective etching of the substrate 120 before or after the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigital transducer (IDT) 130. The IDT 130 includes a first plurality of parallel fingers, such as finger 136, extending from a first busbar 132 and a second plurality of fingers extending from a second busbar 134. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers overlap for a distance AP, commonly referred to as the “aperture” of the IDT. The center-to- center distance L between the outermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites a primary acoustic mode within the piezoelectric plate 110. The primary acoustic mode of an XBAR is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate 110, which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on the diaphragm 115 of the piezoelectric plate which spans, or is suspended over, the cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and length L of the IDT 130. A cavity of an XBAR may have a different shape, such as a regular or irregular polygon. The cavity of an XBAR may have more or fewer than four sides, which may be straight or curved.

The detailed cross-section view (Detail C) shows two IDT fingers 136 a, 136 b on the surface of the piezoelectric plate 110. The dimension p is the “pitch” of the IDT and the dimension w is the width or “mark” of the IDT fingers. A dielectric layer 150 may be formed between and optionally over (see IDT finger 136 a) the IDT fingers. The dielectric layer 150 may be a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. The dielectric layer 150 may be formed of multiple layers of two or more materials. The IDT fingers 136 a and 136 b may be aluminum, copper, beryllium, gold, tungsten, molybdenum, alloys and combinations thereof, or some other conductive material. Thin (relative to the total thickness of the conductors) layers of other metals, such as chromium or titanium, may be formed under and/or over and/or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers and/or to improve power handling. The busbars of the IDT 130 may be made of the same or different materials as the fingers.

For ease of presentation in FIG. 1, the geometric pitch and width of the IDT fingers is greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 110. An XBAR may have hundreds of parallel fingers in the IDT 110. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of a solidly mounted XB AR (SM XBAR) 200. SM XBARs are first described in patent U.S. Pat. No. 10,601,392. The SM XBAR 200 includes a piezoelectric plate 110 and an IDT (of which only fingers 230 and 235 are visible). The piezoelectric layer 110 has parallel front and back surfaces 112, 114. Dimension tp is the thickness of the piezoelectric plate 110. The width of the IDT fingers 230, 235 is dimension m, thickness of the IDT fingers is dimension tm, and the IDT pitch is dimension p.

In contrast to the XBAR device shown in FIG. 1, the IDT of an SM XBAR is not formed on a diaphragm spanning a cavity in a substrate (120 in FIG. 1). Instead, an acoustic Bragg reflector 240 is between a surface 222 of a substrate 220 and the back surface 114 of the piezoelectric plate 110. The acoustic Bragg reflector 240 is both disposed between and mechanically attached to a surface 222 of the substrate 220 and the back surface 114 of the piezoelectric plate 110. In some circumstances, thin layers of additional materials may be disposed between the acoustic Bragg reflector 240 and the surface 222 of the substrate 220 and/or between the Bragg reflector 240 and the back surface 114 of the piezoelectric plate 110. Such additional material layers may be present, for example, to facilitate bonding the piezoelectric plate 110, the acoustic Bragg reflector 240, and the substrate 220.

The acoustic Bragg reflector 240 includes multiple dielectric layers that alternate between materials having high acoustic impedance and materials have low acoustic impedance. “High” and “low” are relative terms. For each layer, the standard for comparison is the adjacent layers. Each “high” acoustic impedance layer has an acoustic impedance higher than that of both the adjacent low acoustic impedance layers. Each “low” acoustic impedance layer has an acoustic impedance lower than that of both the adjacent high acoustic impedance layers. As will be discussed subsequently, the primary acoustic mode in the piezoelectric plate of an XBAR is a shear bulk wave. Each of the layers of the acoustic Bragg reflector 240 has a thickness equal to, or about, one-fourth of the wavelength of a shear bulk wave having the same polarization as the primary acoustic mode at or near a resonance frequency of the SM XBAR 200. Dielectric materials having comparatively low acoustic impedance include silicon dioxide, carbon-containing silicon oxide, and certain plastics such as cross-linked polyphenylene polymers. Materials having comparatively high acoustic impedance include hafnium oxide, silicon nitride, aluminum nitride, silicon carbide, and diamond. All of the high acoustic impedance layers of the acoustic Bragg reflector 240 are not necessarily the same material, and all of the low acoustic impedance layers are not necessarily the same material. In the example of FIG. 2, the acoustic Bragg reflector 240 has a total of six layers. An acoustic Bragg reflector may have more than, or less than, six layers.

As shown in FIG. 2, the IDT fingers 230, 235 have rectangular cross-sections. The IDT fingers 230, 235 may have some other cross-section, such as trapezoidal, T-shaped, or stepped. The IDT fingers 230, 235 are shown as single layer structures which may be aluminum or some other metal. IDT fingers may include multiple layers of materials, which may be selected to have different acoustic loss and/or different acoustic impedance. When multiple material layers are used, the cross-sectional shapes of the layers may be different. Further, a thin adhesion layer of another material, such as titanium or chrome, may be formed between the IDT fingers 230, 235 and the piezoelectric plate 110. Although not shown in FIG. 2, some or all IDT fingers may be disposed in grooves or slots extending partially or completely through the piezoelectric plate 110.

FIG. 3 is a graphical illustration of the primary acoustic mode of interest in an XBAR. FIG. 3 shows a small portion of an XBAR 300 including a piezoelectric plate 310 and three interleaved IDT fingers 330 which alternate in electrical polarity from finger to finger. An RF voltage is applied to the interleaved fingers 330. This voltage creates a time-varying electric field between the fingers. The direction of the electric field is predominantly lateral, or parallel to the surface of the piezoelectric plate 310, as indicated by the arrows labeled “electric field”. Due to the high dielectric constant of the piezoelectric plate, the RF electric energy is highly concentrated inside the plate relative to the air. The lateral electric field introduces shear deformation which couples strongly to a shear primary acoustic mode (at a resonance frequency defined by the acoustic cavity formed by the volume between the two surfaces of the piezoelectric plate) in the piezoelectric plate 310. In this context, “shear deformation” is defined as deformation in which parallel planes in a material remain predominantly parallel and maintain constant separation while translating (within their respective planes) relative to each other. A “shear acoustic mode” is defined as an acoustic vibration mode in a medium that results in shear deformation of the medium. The shear deformations in the XBAR 300 are represented by the curves 360, with the adjacent small arrows providing a schematic indication of the direction and relative magnitude of atomic motion at the resonance frequency. The degree of atomic motion, as well as the thickness of the piezoelectric plate 310, have been greatly exaggerated for ease of visualization. While the atomic motions are predominantly lateral (i.e. horizontal as shown in FIG. 3), the direction of acoustic energy flow of the excited primary acoustic mode is substantially orthogonal to the surface of the piezoelectric plate, as indicated by the arrow 365.

An acoustic resonator based on shear acoustic wave resonances can achieve better performance than current state-of-the art film-bulk-acoustic-resonators (FBAR) and solidly- mounted-resonator bulk-acoustic-wave (SMR BAW) devices where the electric field is applied in the thickness direction. In such devices, the acoustic mode is compressive with atomic motions and the direction of acoustic energy flow in the thickness direction. In addition, the piezoelectric coupling for shear wave XBAR resonances can be high (>20%) compared to other acoustic resonators. High piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters with appreciable bandwidth.

The basic behavior of acoustic resonators, including XBARs, is commonly described using the Butterworth Van Dyke (BVD) circuit model as shown in FIG. 4A. The BVD circuit model consists of a motional arm and a static arm. The motional arm includes a motional inductance L_(m), a motional capacitance C_(m), and a resistance R_(m). The static arm includes a static capacitance C_(o) and a resistance R_(o). While the BVD model does not fully describe the behavior of an acoustic resonator, it does a good job of modeling the two primary resonances that are used to design band-pass filters, duplexers, and multiplexers (multiplexers are filters with more than 2 input or output ports with multiple passbands).

The first primary resonance of the BVD model is the motional resonance caused by the series combination of the motional inductance L_(m) and the motional capacitance C_(m). The second primary resonance of the BVD model is the anti-resonance caused by the combination of the motional inductance Lm, the motional capacitance C_(m), and the static capacitance C_(o). In a lossless resonator (R_(m) =R_(o)=0), the frequency F_(r) of the motional resonance is given by

$\begin{matrix} {F_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{m}}}} & (1) \end{matrix}$

The frequency F_(a) of the anti-resonance is given by

$\begin{matrix} {F_{a} = {F_{r}\sqrt{1 + \frac{1}{\gamma}}}} & (2) \end{matrix}$

where γ=C_(o)/C_(m) is dependent on the resonator structure and the type and the orientation of the crystalline axes of the piezoelectric material.

FIG. 4B is a graph 400 of the magnitude of admittance of a theoretical lossless acoustic resonator. The acoustic resonator has a resonance 412 at a resonance frequency where the admittance of the resonator approaches infinity. In over-simplified terms, the lossless acoustic resonator can be considered a short circuit at the resonance frequency 412 and an open circuit at the anti-resonance frequency 414. The resonance and anti-resonance frequencies in FIG. 4 are representative, and an acoustic resonator may be designed for other frequencies.

FIG. 5 is a schematic circuit diagram of a band-pass filter 500 using five XBARs X1-X5. The filter 500 may be, for example, a band n79 band-pass filter for use in a communication device. The filter 500 has a conventional ladder filter architecture including three series resonators Xl, X3, X5 and two shunt resonators X2, X4. The three series resonators X1, X3, X5 are connected in series between a first port and a second port. In FIG. 5, the first and second ports are labeled “In” and “Out”, respectively. However, the filter 500 is symmetrical and either port may serve as the input or output of the filter. The two shunt resonators X2, X4 are connected from nodes between the series resonators to ground. All the shunt resonators and series resonators are XBARs.

The three series resonators X1, X3, X5 and the two shunt resonators X2, X4 of the filter 500 may be formed on a single plate 530 of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a respective IDT (not shown), with at least the fingers of the IDT disposed over a cavity in the substrate. In this and similar contexts, the term “respective” means “relating things each to each”, which is to say with a one-to-one correspondence. In FIG. 5, the cavities are illustrated schematically as the dashed rectangles (such as the rectangle 535). In this example, an IDT of each resonator is disposed over a respective cavity. In other filters, the IDTs of two or more resonators may be disposed over a common cavity. Resonators may also be divided into multiple sub-resonators with IDTs formed over respective cavities.

Each of the resonators X1 to X5 has a resonance frequency and an anti-resonance frequency. In simplified terms, each resonator is effectively a short circuit at its resonance frequency and effectively an open circuit at its anti-resonance frequency. Each resonator X1 to X5 creates a “transmission zero”, where the transmission between the in and out ports of the filter is very low. Note that the transmission at a “transmission zero” is not actually zero due to energy leakage through parasitic components and other effects. The three series resonators X 1, X3, X5 create transmission zeros at their respective anti-resonance frequencies (where each resonator is effectively an open circuit). The two shunt resonators X2, X4 create transmission zeros at their respective resonance frequencies (where each resonator is effectively a short circuit). In a typical band-pass filter using acoustic resonators, the anti- resonance frequencies of the series resonators are above the passband, and the resonance frequencies of the shunt resonators are below the passband.

The layout of a filter must include conductors that interconnect XBAR devices with the filter. These conductors result in parasitic capacitors and/or inductors that can adversely affect filter performance. In particular, parasitic capacitance between the conductors that connect cascaded sub-resonators (i.e., sub-resonators in series) and ground can create a spur from anti-resonance mode-splitting, which, in turn, may result in a difference in effective anti- resonance frequencies. These anti-resonance frequency offsets may create a local transmission minimum or maximum that can appear as a spur in the filter response. The magnitude of the filter device passband spurs depends on the frequency offset of the effective resonances. Electromagnetic effects that can lead to the creation of these die layout-induced spurs must be considered to prevent or reduce these effects.

FIG. 6A is a schematic diagram of a conventional layout of a portion of an XBAR 600 with two sub-resonators 613, 615 in series. Sub-resonators 613 and 615 are interconnected to by conductor 617, sub-resonator 613 is connected to other portions of the XBAR 600 or other portions of a filter circuit via conductor 619, and sub-resonator 615 is connected to other portions of the XBAR 600 or other elements of a filter circuit via conductor 621. FIG. 6B is a schematic BVD circuit diagram 650 of the portion of XBAR 600 of FIG. 6A. Interconnect capacitance between the conductor 617 and ground (Gnd) is shown as C_(i).

FIG. 7 is a graph 700 of the simulated performance of the conventional circuit with the two sub-resonators of FIGS. 6A and 6B, where the magnitude of admittance is shown as a function of frequency. Dashed curve 720 shows the magnitude of admittance for this circuit with an idealized capacitance to ground C_(i)=0. Solid curve 710 shows the magnitude of admittance for this circuit with an exemplary capacitance to ground of C_(i)/C_(o) =0.01, where C_(o) is the static capacitance of the sub-resonators 613, 615. The effect of the non-zero capacitance to ground is a spur 730 adjacent the anti-resonance frequency. This spur potentially negatively impacts the performance of a filter circuit using this XBAR.

FIG. 8 is a schematic circuit diagram of a third-order bandpass filter including series resonators 813 and 815 and a shunt resonator made up of sub-resonators 817 and 819. Interconnect capacitance to ground (Gnd) for the conductor interconnecting sub-resonators 817 and 819 is shown as C_(i). The filter 800 shown is simplified and only depicts three resonators. However, filters devices typically have more than three resonators, each of which may be divided into two or more sub-resonators.

FIG. 9 is a graph 900 of the simulated performance of the filter 800 of FIG. 8, where a transfer function through the filter (i.e., S21) is shown as a function of frequency. Dashed curve 920 shows S21 for this circuit with an idealized capacitance to ground C_(i)=0. Solid curve 910 shows the S21 for this circuit with C_(i)/C₀=0.1. The effect of the non-zero capacitance to ground is a spur 930. This spur potentially negatively impacts the performance of the filter. The dotted line 930 is a plot of S21 for this circuit with C_(i)/C₀=0.02. C_(i)≤0.02C₀ may be sufficient to prevent objectionable spurs in the filter transfer function S21.

FIG. 10 is a schematic diagram of an improved layout of a portion of an XBAR 1000 with sub-resonators 1013 and 1015 in series. A conductor 1050 connecting sub- resonators 1013 and 1015 has a shape that minimizes an area of the conductor. For example, the conductor 1050 shown in FIG. 10 has a bowtie shape, where a bowtie shape has a narrower middle portion and wider lateral portions. The conductor 1050 shown in FIG. 10 includes three portions, a first portion 1054 between a second portion 1052 adjacent sub-resonator 1013 and a third portion 1056 adjacent sub-resonator 1015, wherein a width (as shown in a vertical direction in FIG. 10) of the second portion 1052 decreases linearly from sub-resonator 1013 to the first portion 1054 and a width of the third portion 1056 decreases linearly from sub- resonator 1015 to the first portion 1054. In this example, a width of the first portion 1054 is constant.

A capacitance to ground from the conductor 1050 connecting sub-resonators 1013 and 1015 is sufficiently small such that the performance of the device is not negatively impacted. For example, in certain devices when C_(i)≤0.02C₀, the performance of the device would not be negatively impacted. As a result, the capacitance to ground would not result in the spurs that negatively impact performance of the device seen in FIGS. 7 and 9.

While the conductor 1050 shown in FIG. 10 has a bowtie shape, the conductor interconnecting sub-resonators in series can have other suitable shapes that minimize or reduce an area of the conductor. The area of the conductor can be minimized to have the smallest possible area that sufficiently conducts between the sub-resonators such that the performance of the device is not negatively impacted and the device can be manufactured using typical manufacturing methods. For example, the first portion 1054, the second portion 1052 and the third portion 1056 are shown to have straight sides, but could have one or more curved portions. The first portion 1054 is shown to have a constant width, but could have a width that varies. Each of the portions could be formed of one or more metal layers or portions, each having a different shape. The lengths (as shown in a horizontal direction in FIG. 10) of each of the portions are shown to be roughly equal, but could be different with respect to each other. The conductor 1050 is shown to be roughly symmetric, but could be asymmetric.

Description of Methods

FIG. 11 is a simplified flow chart showing a process 1100 for making an XBAR or a filter incorporating XBARs, e.g., sub-resonators in series between a first node and a second node. The process 1100 starts at 1105 with a substrate 1104 and a plate of piezoelectric material 1102 and ends at 1195 with a completed XBAR or filter. The flow chart of FIG. 11 includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in FIG. 11.

The flow chart of FIG. 11 captures three variations of the process 1100 for making an XBAR or filter device which differ in when and how cavities are formed in the substrate. The cavities may be formed at steps 1110A, 1110B, or 1110C. Only one of these steps is performed in each of the three variations of the process 1100.

The piezoelectric plate may be, for example, Z-cut lithium niobate or lithium tantalate with Euler angles 0, 0, 90° . The piezoelectric plate may be rotated Z-cut lithium niobate with Euler angles 0, β, 90° , where βis in the range from −15° to +5° . The piezoelectric plate may be rotated Y-cut lithium niobate or lithium tantalate with Euler angles 0, (β, 0, where βis in the range from 0 to 60° . The piezoelectric plate may be some other material or crystallographic orientation. The substrate may preferably be silicon. The substrate may be some other material that allows formation of deep cavities by etching or other processing.

In one variation of the process 1100, one or more cavities are formed in the substrate at 1110A, before the piezoelectric plate is bonded to the substrate at 1120. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using conventional photolithographic and etching techniques. Typically, the cavities formed at 1110A will not penetrate through the substrate.

At 1120, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and the substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the substrate and the piezoelectric plate are highly polished. One or more layers of intermediate materials, such as an oxide or metal, may be formed or deposited on the mating surface of one or both of the piezoelectric plate and the substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the substrate or intermediate material layers.

A conductor pattern, including IDTs of each XBAR, is formed at 1130 by depositing and patterning one or more conductor levels on the front side of the piezoelectric plate. The conductor levels typically include a first conductor level that includes the IDT fingers, and a second conductor level formed over the IDT busbars and other conductors except the IDT fingers. In some devices, a third conductor levels may be formed on the contact pads. Each conductor level may be one or more layers of, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between each conductor layer and the piezoelectric plate) and/or on top of each conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the first conductor level and the piezoelectric plate. The second conductor level may be a conduction enhancement layer of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the first conductor level (for example the IDT bus bars and interconnections between the IDTs).

Each conductor level may be formed at 1130 by depositing the appropriate conductor layers in sequence over the surface of the piezoelectric plate. The excess metal may then be removed by etching through patterned photoresist. The conductor level can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

Alternatively, each conductor level may be formed at 1130 using a lift-off process. Photoresist may be deposited over the piezoelectric plate. and patterned to define the conductor level. The appropriate conductor layers may be deposited in sequence over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor level.

When a conductor level has multiple layers, the layers may be deposited and patterned separately. In particular, different patterning processes (i.e. etching or lift-off) may be used on different layers and/or levels and different masks are required where two or more layers of the same conductor level have different widths or shapes.

The conductors can be configured such that an area of conductors between sub- resonators in series is minimized or reduced.

At 1140, dielectric layers may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. As previously described, the dielectric layers may include a different dielectric thickness over the IDT fingers of the XBARs within each sub-filter. Each dielectric layer may be deposited using a conventional deposition technique such as sputtering, evaporation, or chemical vapor deposition. Each dielectric layer may be deposited over the entire surface of the piezoelectric plate, including on top of the conductor pattern. Alternatively, one or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric plate, such as only between the interleaved fingers of the IDTs. Masks may also be used to allow deposition of different thicknesses of dielectric materials on different portions of the piezoelectric plate.

In a second variation of the process 1100, one or more cavities are formed in the back side of the substrate at 1110B. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using an anisotropic or orientation- dependent dry or wet etch to open holes through the back side of the substrate to the piezoelectric plate. In this case, the resulting resonator devices will have a cross-section as shown in FIG. 1.

In a third variation of the process 1100, one or more cavities in the form of recesses in the substrate may be formed at 1110C by etching the substrate using an etchant introduced through openings in the piezoelectric plate. A separate cavity may be formed for each resonator in a filter device.

In all variations of the process 1100, the filter device is completed at 1160. Other actions that may occur at 1160 include depositing an encapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or a portion of the device; forming bonding pads or solder bumps or other means for making connection between the device and external circuitry; excising individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at 1160 is to tune the resonant frequencies of the resonators within the device by adding or removing metal or dielectric material from the front side of the device. After the filter device is completed, the process ends at 1195.

FIG. 12 is a simplified flow chart showing another process 1200 for making an XBAR or a filter incorporating XBARs, e.g., sub-resonators in series between a first node and a second node. The process 1200 starts at 1205 with a device substrate 1204 and a piezoelectric plate on a sacrificial substrate, and ends at 1295 with a completed XBAR or filter. The flow chart of FIG. 12 includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in FIG. 12.

The piezoelectric plate may be, for example, Z-cut lithium niobate or lithium tantalate with Euler angles 0, 0, 90° . The piezoelectric plate may be rotated Z-cut lithium niobate with Euler angles 0, β, 90° , where βis in the range from -15° to +5° . The piezoelectric plate may be rotated Y-cut lithium niobate or lithium tantalate with Euler angles 0, (β, 0, where βis in the range from 0 to 60° . The piezoelectric plate may be some other material or crystallographic orientation. The substrate may preferably be silicon. The substrate may be some other material that allows formation of deep cavities by etching or other processing.

At 1210, an acoustic Bragg reflector is formed by depositing alternating dielectric layers of high acoustic impedance and low acoustic impedance materials. Each of the layers has a thickness equal to or about one-fourth of the acoustic wavelength. Dielectric materials having comparatively low acoustic impedance include silicon dioxide, carbon-containing silicon oxide, and certain plastics such as cross-linked polyphenylene polymers. Dielectric materials having comparatively high acoustic impedance include silicon nitride and aluminum nitride. All of the high acoustic impedance layers are not necessarily the same material, and all of the low acoustic impedance layers are not necessarily the same material. The total number of layers in the acoustic Bragg reflector may be from about five to more than twenty.

At 1210, all of the layers of the acoustic Bragg reflector may be deposited on either the surface of the piezoelectric plate on the sacrificial substrate 1202 or a surface of the device substrate 1204. Alternatively, some of the layers of the acoustic Bragg reflector may be deposited on the surface of the piezoelectric plate on the sacrificial substrate 1202 and the remaining layers of the acoustic Bragg reflector may be deposited on a surface of the device substrate 1204.

At 1220, the piezoelectric plate on the sacrificial substrate 1202 and the device substrate 1204 may be bonded such that the layers of the acoustic Bragg reflector are between the piezoelectric plate and the device substrate. The piezoelectric plate on the sacrificial substrate 1202 and the device substrate 1204 may be bonded using a wafer bonding process such as direct bonding, surface-activated or plasma-activated bonding, electrostatic bonding, or some other bonding technique. Note that, when one or more layers of the acoustic Bragg reflector are deposited on both the piezoelectric plate and the device substrate, the bonding will occur between or within layers of the acoustic Bragg reflector.

After the piezoelectric plate on the sacrificial substrate 1202 and the device substrate 1204 may be bonded, the sacrificial substrate, and any intervening layers, are removed at 1230 to expose the surface of the piezoelectric plate (the surface that previously faced the sacrificial substrate). The sacrificial substrate may be removed, for example, by material-dependent wet or dry etching or some other process.

A conductor pattern, including IDTs of each XBAR, is formed at 1240 by depositing and patterning conductor levels on the front side of the piezoelectric plate. The conductor levels typically include a first conductor level that includes the IDT fingers, and a second conductor level formed over the IDT busbars and other conductors except the IDT fingers. In some devices, a third conductor levels may be formed on the contact pads. Each conductor level may be one or more layers of, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between each conductor layer and the piezoelectric plate) and/or on top of each conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the first conductor level and the piezoelectric plate. The second conductor level may be a conduction enhancement layer of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the first conductor level (for example the IDT bus bars and interconnections between the IDTs).

Each conductor level may be formed at 1240 by depositing the appropriate conductor layers in sequence over the surface of the piezoelectric plate. The excess metal may then be removed by etching through patterned photoresist. The conductor level can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

Alternatively, each conductor level may be formed at 1240 using a lift-off process. Photoresist may be deposited over the piezoelectric plate. and patterned to define the conductor level. The appropriate conductor layers may be deposited in sequence over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor level.

When a conductor level has multiple layers, the layers may be deposited and patterned separately. In particular, different patterning processes (i.e. etching or lift-off) may be used on different layers and/or levels and different masks are required where two or more layers of the same conductor level have different widths or shapes.

The conductors can be configured such that an area of conductors between sub- resonators in series is minimized or reduced.

At 1250, dielectric layers may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. As previously described, the dielectric layers may include a different dielectric thickness over the IDT fingers of the XBARs within each sub-filter. Each dielectric layer may be deposited using a conventional deposition technique such as sputtering, evaporation, or chemical vapor deposition. Each dielectric layer may be deposited over the entire surface of the piezoelectric plate, including on top of the conductor pattern. Alternatively, one or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric plate, such as only between the interleaved fingers of the IDTs. Masks may also be used to allow deposition of different thicknesses of dielectric materials on different portions of the piezoelectric plate.

In all variations of the process 1200, the filter device is completed at 1260. Other actions that may occur at 1260 include depositing an encapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or a portion of the device; forming bonding pads or solder bumps or other means for making connection between the device and external circuitry; excising individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at 1260 is to tune the resonant frequencies of the resonators within the device by adding or removing metal or dielectric material from the front side of the device. After the filter device is completed, the process ends at 1295.

Closing Comments

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of ”and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items. 

It is claimed:
 1. An acoustic filter device comprising: a transversely-excited film bulk acoustic resonator (XBAR) comprising a plurality of sub-resonators; and conductors connecting the plurality of sub-resonators in series between a first node and a second node, wherein at least one of the conductors connects two of the plurality of sub-resonators and has a shape that minimizes an area of the at least one conductor.
 2. The device of claim 1, wherein the shape is a bowtie.
 3. The device of claim 1, wherein the two of the plurality of sub-resonators comprise a first sub-resonator and a second sub-resonator, wherein the at least one conductor comprises three portions, a first portion between a second portion adjacent the first sub-resonator and a third portion adjacent the second sub- resonator, and wherein a width of the second portion decreases from the first sub-resonator to the first portion and a width of the third portion decreases from the second sub-resonator to the first portion.
 4. The device of claim 3, wherein a width of the first portion is constant.
 5. The device of claim 1, wherein the XBAR comprises a piezoelectric plate.
 6. The device of claim 5, wherein each of the plurality of sub-resonators comprises a respective portion of the piezoelectric plate spanning a cavity to form a respective diaphragm and a respective interdigital transducer (IDT), interleaved fingers of the respective IDT on the respective diaphragm.
 7. The device of claim 5, wherein the piezoelectric plate is solidly mounted to a substrate with a Bragg reflector between the piezoelectric plate and the substrate.
 8. The device of claim 5, wherein each sub-resonator comprises a plurality of interleaved fingers on the piezoelectric plate.
 9. The device of claim 1, wherein C_(i) is capacitance between the at least one of the conductors connecting the two of the plurality of sub-resonators and ground, C_(o) is static capacitance of the two of the plurality of sub-resonators, and C_(i) is less than or equal to 0.02C_(o).
 10. The device of claim 1, wherein a resonant frequency of a first of the two of the multiple sub-resonators is offset from a second two of the multiple sub-resonators.
 11. An acoustic filter device comprising: a transversely-excited film bulk acoustic resonator (XBAR) comprising a plurality of sub-resonators; and conductors connecting the plurality of sub-resonators in series between a first node and a second node, wherein a capacitance to ground from at least one of the conductors connecting two of the plurality of sub-resonators is sufficiently small.
 12. The device of claim 11, wherein the at least one conductor connecting the two of the multiple sub-resonators has a shape that minimizes an area of the at least one conductor.
 13. The device of claim 12, wherein the shape is a bowtie.
 14. The device of claim 12, wherein the two of the plurality of sub-resonators comprise a first sub-resonator and a second sub-resonator, wherein the at least one conductor comprises three portions, a first portion between a second portion adjacent the first sub-resonator and a third portion adjacent the second sub- resonator, and wherein a width of the second portion decreases from the first sub-resonator to the first portion and a width of the third portion decreases from the second sub-resonator to the first portion.
 15. The device of claim 14, wherein a width of the first portion is constant.
 16. The device of claim 11, wherein the XBAR comprises a piezoelectric plate.
 17. The device of claim 16, wherein each of the plurality of sub-resonators comprises a respective portion of the piezoelectric plate spanning a cavity to form a respective diaphragm and a respective interdigital transducer (IDT), interleaved fingers of the respective IDT on the respective diaphragm.
 18. The device of claim 16, wherein the piezoelectric plate is solidly mounted to a substrate with a Bragg reflector between the piezoelectric plate and the substrate.
 19. The device of claim 16, wherein each sub-resonator comprises a plurality of interleaved fingers on the piezoelectric plate.
 20. The device of claim 11, wherein a resonant frequency of a first of the two of the multiple sub-resonators is offset from a second two of the multiple sub-resonators. 